This invention relates generally to the field of absorption chillers, and more particularly to a feedforward control for an absorption chiller.
Absorption chillers differ from mechanical vapor compression chillers in that they utilize a thermal or chemical process to produce the refrigeration effect necessary to provide chilled water. There is no mechanical compression of the refrigerant taking place within the machine as occurs within more traditional vapor compression type chillers. Most commercial absorption chillers use lithium bromide (a salt) and water as the fluid pair, with lithium bromide being the absorbent and water being the refrigerant. In order to produce the refrigeration effect necessary to make, for example, 44 F chilled water, the shellside of the machine must be maintained in a deep vacuum to allow the refrigerant (water) to boil at approximately 40 F. The lithium bromide solution absorbs the vaporized refrigerant, diluting it before it is pumped to the generator section of the machine where heat is added to reconcentrate the dilute solution. The water vapor boiled off in the generator is then condensed, returning to the evaporator as liquid. The reconcentrated lithium bromide returns to the absorber section as a strong solution to begin the cycle again.
In an absorption chiller, the chilled water temperature is directly affected by disturbances such as the cooling water temperature and the entering chilled water temperature. These disturbances are slowly removed by existing capacity control systems slowly as a result of the slow machine dynamics that exist between the burner and the leaving chilled water temperature. The result is poor transient temperature regulation of the leaving chilled water temperature.
Briefly stated, a feedforward control method for an absorption chiller includes determining the disturbance transfer function, determining the capacity valve transfer function, measuring the actual disturbance, and implementing the feedforward control function in a feedforward controller. The feedforward control function is represented by the ratio of the disturbance transfer function divided by the capacity valve transfer function. The disturbance transfer function and the capacity valve transfer function are measured by applying a known amplitude input perturbation to the disturbance or capacity valve and recording the resulting perturbation in the output leaving chilled water temp. The disturbance transfer function is then the ratio of the delta leaving chilled water temperature divided by the delta change in the disturbance. The capacity transfer function is the ratio of the delta leaving chilled water temperature divided by the delta change in the capacity valve.
According to an embodiment of the invention, a feedforward control method for an absorption chiller system, wherein a control input for said chiller is a heat source controlled by a capacity valve, and wherein said capacity valve is controlled by a feedforward controller, includes the steps of (a) determining a disturbance transfer function between a measured disturbance input into said system and a leaving chilled water output; (b) determining a capacity valve transfer function between said capacity valve and said leaving chilled water output; (c) measuring an actual disturbance introduced into said system; and (d) controlling said capacity valve based on a feedforward control transfer function in said feedforward controller, wherein said feedforward control transfer function is represented by a ratio of said disturbance transfer function to said capacity valve transfer function.
According to an embodiment of the invention, a feedforward controller for an absorption chiller system, wherein a control input for said chiller is a heat source controlled by a capacity valve, and a system output is a leaving chilled water temperature, includes a feedforward loop having a feedforward control transfer function represented by a ratio of a disturbance transfer function to a capacity valve transfer function, wherein said feedforward control transfer function receives a disturbance input as an input; a first summer receiving both a set point input and a feedback from a system transfer function as inputs, and sending an output to a capacity control transfer function; a second summer receiving an output from said capacity control transfer function and an output from said feedforward control transfer function, and sending an output to said system transfer function; said system transfer function receiving said output from said second summer as an input, and also receiving said disturbance input as an input, wherein an output of said system transfer function is said leaving chilled water temperature.
According to an embodiment of the invention, a feedforward control for an absorption chiller system, wherein a control input for said chiller is a heat source controlled by a capacity valve, and wherein said capacity valve is controlled by a feedforward controller, includes means for determining a disturbance transfer function between a measured disturbance input into said system and a leaving chilled water output; means for determining a capacity valve transfer function between said capacity valve and said leaving chilled water output; means for measuring an actual disturbance introduced into said system; and means for controlling said capacity valve based on a feedforward control transfer function in said feedforward controller, wherein said feedforward control transfer function is represented by a ratio of said disturbance transfer function to said capacity valve transfer function.